"Ensure your Nintendo DSi is connected to a power source or has at least 50% battery life before beginning the firmware installation. Powering off during an update may result in a bricked system."
: These allow for emulation, region-free gaming, and backing up your physical cartridges. Nintendo Dsi Firmware Download
While downloading and installing custom firmware is generally considered a legal grey area (often falling under interoperability rights), downloading proprietary Nintendo firmware files (like the official BIOS or DSiWare games you do not own) constitutes piracy and is illegal. "Ensure your Nintendo DSi is connected to a